This invention relates to a novel resin that may be used as a binder in water reducible coating compositions to provide a unique combination of film toughness and flexibility. The invention also includes coating compositions containing the resin. This type of coating composition is particularly useful for coating metal substrates such as metal food and beverage cans.
Polymers of ethylenically unsaturated monomers typically form thermoplastic polymers that are characterized by toughness, but which produce unsuitably brittle films. It is known to offset the brittleness by adding plasticizers, but the improved film flexibility is often at the expense of some of the toughness. These thermoplastic polymers of ethylenically unsaturated monomers sometimes have relatively high glass transition temperatures (T.sub.g), and for some coating applications it is desirable to provide the property of high T.sub.g in the binder resin. Therefore it would be desirable for a number of reasons to include more of a thermoplastic component in some coatings.
In addition to the drawback of brittleness, another problem with incorporating thermoplastics into coating compositions is the difficulty of solubilizing substantial amounts of the thermoplastic polymers in aqueous medium. Since waterborne coating compositions are becoming increasingly desirable for environmental reasons, it would be very advantageous to be able to include thermoplastic polymers in waterborne compositions. It has been proposed to copolymerize ethylenically unsaturated monomers with solubilizable groups so as to yield water reducible copolymers, but the resulting copolymers are generally not thermoplastic in nature.